This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-115301, filed Apr. 17, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to a mobile communication system which enables radio communication between base stations with directional antenna apparatuses and mobile station and to base stations used for the system.
In this type of mobile communication system, because of effective use of frequency, suppression of the multipath fading, and elimination of cochannel interference waves, directional antennas have been used at base stations in order to point directivity only toward a mobile station.
A mobile communication system of this type disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-145286 is composed of a control station 1 and a base station 2 as shown in FIG. 13. The control station 1 includes a modulator 3 and a demodulator 4, both connected to a public telecommunication network, a local oscillator 5, a mixer 6, a circulator, a power feed distributor 7, a variable attenuator, a phase shifter, a wavelength multiplexer 8. The base station 2 includes a wavelength multiplexer 10 connected to an optical fiber 9, a local oscillator 11, an antenna element driver 12, and an antenna element 13. The mobile communication system enables a two-way wireless communication between the base station 2 with an array antenna composed of a plurality of antenna elements and a mobile station. The array antenna acts as a variable directional antenna whose directivity varies according to the electric signal fed to the antenna.
The signal received by the array antenna at the base station 2 from a mobile station is sent to the control station 1. The control station 1 estimates the directions of arrival of the radiowave from the signal received from the mobile station, calculates the electric signal to be fed to the array antenna of the base station 2, and supplies the electric signal to the array antenna. The directivity of the array antenna is varied so as to point toward the position of the mobile station.
The directions of arrival of the radiowave is estimated by a method of causing the beam to scan and sensing the direction from the reception level, or an directions of arrival estimating method, such as MUSIC or ESPRIT. Since the directions of arrival of the radiowave varies constantly as the mobile station moves, the directions of arrival of the radiowave is estimated at suitable intervals of time and the electric signal to be fed to the array antenna of the base station is calculated on the basis of the estimation.
With the mobile communication system disclosed in the above publication, however, the computing process takes time, causing a problem: when a mobile station moves fast, the base station cannot track the mobile station, resulting in a break in the communication.
The object of the present invention is to provide a directional antenna apparatus capable of causing the directivity of a base station to keep reliable track of even a fast moving mobile station and constantly assuring stable radio communication between the base station and the mobile station, a mobile communication system using the directional antenna apparatus, and a base station used for the system.
According to the invention according to claim 1, there is provided a mobile communication system comprising: a plurality of antenna elements which transmit and receive a radio signal to and from a mobile station; weighting means for weighting the amplitude and phase of the transmission or reception signal of each of the individual antenna elements; adding means for adding the reception signals from the individual antenna elements weighted by the weighting means and outputting the resulting signal to a reception unit; dividing means for dividing a transmission signal from a transmission unit to the weighting means corresponds to the individual antenna elements; storage means for storing weighting coefficients suitable for communication with the mobile station for each of a plurality of moving positions of the mobile station; and control means for setting the weighting coefficients stored in the storage means in the weighting means each timing the mobile station moves to each of the moving positions.
With the present invention, a base station senses the direction of a mobile station, while the mobile station is moving, need not do calculations to point the directivity to the direction of the mobile station, and causes the directivity to track the mobile station reliably even when the mobile station moves fast.
Consequently, it is possible to provide a mobile communication system which enables reliable radio communication constantly between a base station and a mobile station.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.